The high viscosity of many petroleum liquids is a factor contributing significantly to the underutilization of these valuable natural resources. Viscosity greatly complicates, and may even defeat, the extraction of many types of petroleum from the Earth. Viscosity remains a concern following extraction, as high viscosity significantly hampers the pumping, transportation, refining and handling of petroleum. Examples of viscous petroleum that can only be extracted and/or refined with great difficulty include Cerro Negro heavy crude oil, Orinoco heavy crude oil, and bitumens such as Athabascan tar. Because of this, the petroleum industry has long recognized the need for a safe, economical and effective method for reducing the viscosity of valuable fossil fuel resources.
Under certain circumstances, standard refining processes such as hydrotreating or hydrodesulfurization (HDS) can favorably affect the viscosity of petroleum liquids during refining. Hydrotreatment involves the exposure of petroleum to hydrogen gas in the presence of a metal catalyst, under conditions of elevated temperature and pressure. Reduced viscosity, when observed, is largely the result of the increased temperature to which the petroleum refining fraction is exposed. Some reduction in viscosity is also achieved through the breakdown of complex hydrocarbons (e.g., aromatic hydrocarbons), into simpler hydrocarbons of low molecular weight. Hydrotreating is not a refining process that was developed for the reduction of petroleum viscosity, however, and viscosity reduction is only an incidental side benefit that is occasionally observed and is poorly controllable or reproducible. Furthermore, hydrotreating is useful only at a limited number of steps in the refining of petroleum, due to the extreme conditions involved and the specialized containment and safety equiptment required.
A more generally accepted and controllable method of modulating petroleum viscosity during refining involves diluting viscous petroleum liquids with low viscosity petroleum refining fractions, usually light-end distillates. Light-end distillates that are used as viscosity lowering diluents are referred to as cutter fractions. Thus, the viscosity of heavy crude oil or bitumen is lowered by "cutting" it with such a light-end distillate. This technique is useful at some stages of the petroleum refining process, but is not economical for large-scale use, or to assist with the extraction of viscous petroleum liquids from the earth.
Other techniques which have been developed for the control of high viscosity in petroleum liquids center on the use of chemical additives such as the tensioactive compounds (surfactants) described by R. S. Scheffee in U.S. Pat. No. 4,498,906. Scheffee discloses a combustible formulation in which surfactants of biological origin are added at the close of the refining process. The use of such chemical additives at earlier stages of the extraction and refining of petroleum is prohibited by expense and by the need to remove surfactants in order to subject the petroleum to certain standard refining procedures. Surfactant recovery greatly complicates the refining process.
Various investigators, e.g., Bertrand et al. (1983), 5 BIOTECH. LETT. (No. 8) 567-572, have described the production of biosurfactants in situ by microbial organisms grown in the presence of crude oil. These biosurfactants assist in the dispersal of crude oil in seawater, thus facilitating the bioremediation of oil spills and chronic petroleum pollution. Microorganisms used for bioremediation purposes, however, are not generally compatible with petroleum extraction and refining processes, because they also attack and catabolize (destroy) combustible hydrocarbons.
Other types of microorganisms have been used for the relatively controlled destruction of certain compounds in petroleum, with the result that viscosity of the treated product is stabilized. For example, D. O. Hitzmann in U.S. Pat. No. 3,069,325 describes a method for stabilizing the viscosity of jet fuels when stored, as in military installations, over seawater. The viscosity of untreated jet fuels increases upon exposure to such storage conditions, due to the accumulation of a sludge composed of natural bacteria present in the seawater that attack the stored fuel as a source of nutrients such as carbon, nitrogen, and sulfur. This sludge becomes dispersed within the fuel upon physical agitation, increasing its viscosity and therefore the risk of clogging jet engines. Hitzmann prevents the accumulation of this sludge by pretreatment of jet fuel with strains of bacteria adapted to the consumption of nitrogen- or sulfur-containing hydrocarbon fuel molecules as nutrient sources. When the nutrient-depleted fuel is thereafter stored over seawater, naturally occurring bacteria are unable to attack it as a metabolic substrate. Viscosity remains stable upon storage, but this result is achieved through the loss of combustible sulfur- and nitrogen-containing hydrocarbons. The Hitzmann method is thus unsuitable for use prior to or during the refining process, as it destroys a portion of the caloric value of the treated fuel.
Still other types of microorganisms have been described as usful in other aspects of petroleum refining, e.g., desulfurization. Virtually all of these function as in the Hitzmann process, by the controlled demolition of certain types of troublesome compounds found in petroleum. Microorganisms such as Thiobacillus ferrooxidans have been harnessed to the removal of pyritic sulfur, while others such as Pseudomonas putida have been investigated for the removal of organic sulfur from petroleum. Microbial desulfurization technology is reviewed in Monticello and Finnerty (1985), 39 ANN. REV. MICROBIOL. 371-389, and Bhadra et al. (1987), 5 BIOTECH. ADV. 1-27. Kilbane (1989), 7 TRENDS BIOTECHNOL. (No. 4) 97-101 provides commentary on recent developments in the field. Such desulfurizing microorganisms are not thought of by those involved in petroleum extraction and refining as effective in altering the physical properties, e.g., viscosity, of petroleum. Indeed, the ineffectiveness of such microorganisms for the control of viscosity is illustrated by E. P. Kopacz in U.S. Pat. No. 4,632,906: attempts to desulfurize a vacuum residual oil using Bacillus sulfasportare ATCC No. 39909 were ineffective or poorly effective, until the viscous petroleum liquid was cut with a light petroleum oil cutter fraction (column 4, lines 23-25). Moreover, desulfurizing microorganisms are used for the treatment of many types of fossil fuels for which viscosity is irrelevant (e.g., solids such as coal), arises from complex and diverse phenomena, or is already sufficiently low to prevent no obstacle to refining procedures (e.g., middle distillates such as FCC light cycle oil or No. 1 diesel fuel).
A need remains for a viscosity reducing treatment that can be used to facilitate the handling of viscous petroleum liquids at any desired stage of the extraction and/or refining processes. A suitable viscosity reducing treatment would not require specialized equipment or safety procedures, and would not degrade the caloric (fuel) value of the treated petroleum.